1. Field of the Invention
The present invention relates to a controller for a power steering apparatus which controls assist torque produced by the power steering apparatus.
2. Discussion of Related Art
In conventional power steering apparatus, pressurized fluid is utilized to generate and control assist torque. Further, there has been proposed a power steering apparatus in which pressurized fluid is supplied by a hydraulic pump driven by an electric motor. In such a power steering apparatus, called "motor-driven pump type", has an advantage in that the rotational speed of the pump can freely be controlled. In order to control the power steering apparatus of the motor-driven pump type, there has been proposed a feed back control in which the drive voltage supplied to the electric motor of the pump is continuously detected, and the drive voltage is controlled by PI (proportional plus integral) control based on the detected drive voltage so that the drive voltage follows a target drive voltage.
However, in such a feed back control, extraordinary cases sometime arise where the difference between a target voltage and the detected drive voltage becomes as large as the target voltage. For example, the difference becomes as large as the target voltage when an abrupt variation occurs in the drive circuit immediately after the power switch is turned on. For quickly reducing the difference, the capacity of compensation, i.e., the gain of a drive circuit for driving the motor, must be set to be very high. However, if the gain of the drive circuit is set to be very high, the stability in an ordinary state is deteriorated. In detail, even when the drive voltage is slightly deviated from a target voltage due to disturbance, a compensation voltage whose magnitude is almost equal to that of the target voltage is produced to compensate the difference. Such excessive compensation is disadvantageous in drive circuits for a motor-driven pump, in which a stability is regarded as an important factor to obtain a proper ability of following (hereinafter referred to as "followability").
Further, in cases where the feed back function of the drive circuit is stopped due to a malfunction of the drive circuit, the difference becomes large due to an abnormal feed back signal, even though the target voltage is properly calculated and output. In such a case, the rotational speed of the motor-driven pump cannot be controlled properly. This brings about a fear that assist torque is not properly generated by the power steering apparatus and also a problem that energy may be wastefully consumed by the motor.
Moreover, when it is tried to increase the following rate of the drive voltage with respect to a target voltage, namely, the responsiveness in the conventional controller utilizing PI control, a control value, according to which the motor is driven, suffers from overshoots, as is well known in the control theory. Due to such overshoots, the control value cannot follow the target voltage in cases where the target voltage frequently changes, resulting in deterioration of the control accuracy. Accordingly, in order to make the operation of the controller stable, the responsiveness must be low. However, the responsiveness cannot be sacrificed in the control for motor-driven pumps, because a high responsiveness is required to quickly change assist torque. Therefore, it has been desired to develop a controller for motor-driven pumps which can increase the control accuracy without sacrificing the responsiveness.